Calling Roppongi the red light district of Tokyo does not seem right to me. There are a lot of red light districts in Tokyo, but I Roppongi is more than a Red Light district. While Roppongi has its fair share of prostitution, ranging from soup lands to high class SM clubs, its also well known as a place were foreigners hang out, and expats on a higher budget live. ( E.g. most of the US Embassy staff usually lives in Roppongi or the neighbooring districts) .
In brief, Roppongi is an area were a lot of well off foreigners live, there are quite a few foreign companies there, its close to one of the embassy districts, and if you read "Roppongi" in a job ad., read it as a place were foreigners who don't speak Japanese feel a little better, than in other districts of Tokyo.